PARIS — More than a million people joined over 40 presidents and prime ministers on the streets of Paris on Sunday in the most striking show of solidarity in the West against the threat of Islamic extremism since the Sept. 11 attacks.

Responding to terrorist strikes that killed 17 people in France and riveted worldwide attention, Jews, Muslims, Christians, atheists and people of all races, ages and political stripes swarmed central Paris beneath a bright blue sky, calling for peace and an end to violent extremism.

The Interior Ministry described the demonstration as the largest in modern French history, with as many as 1.6 million people. Many waved the tricolor French flag and brandished pens in raised fists to commemorate those killed Wednesday in an attack on the satirical newspaper Charlie Hebdo, as well as four others killed at a Jewish supermarket on Friday. Thousands hoisted black and white signs bearing three words that have ricocheted through social media as a slogan of unity and defiance: “Je suis Charlie.”

The crowd included Pascale Trager-Lewis, 45, a lawyer, and her husband, Christian Chevalier, 45, who brought their two daughters because they wanted them to witness a historic event. “We came because my husband is an authentic French person; I am Jewish,” Ms. Trager-Lewis said. “My elder daughter’s godmother is a Muslim, and my closest friend almost became a nun. I came for the Jews who were killed, for the freedom of speech, for religious tolerance.”

The world leaders — including President François Hollande of France, Chancellor Angela Merkel of Germany, Prime Minister Benjamin Netanyahu of Israel, President Mahmoud Abbas of the Palestinian Authority and Prime Minister David Cameron of Britain — joined the march in a solemn line. They moved slowly, clasping arms to show solidarity with the victims. The crowd roared in approval.

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Scenes From Paris Rally Against Violence

More than a million people marched in Paris to honor victims of the deadly attacks on the satirical newspaper Charlie Hebdo and on a kosher supermarket.

Earlier in the day the French Interior Ministry held what it described as a security summit meeting, bringing together top intelligence and law enforcement officials from across Europe and North America to discuss ways to prevent terrorism.

Attorney General Eric H. Holder Jr. attended the meeting and announced that the White House would convene an international forum on Feb. 18 to discuss new means of countering terrorism. The White House, in a statement, said the meeting would address domestic and international measures “to prevent violent extremists and their supporters from radicalizing, recruiting or inspiring individuals or groups in the United States and abroad to commit acts of violence.”

Mr. Holder did not participate in the rally and march; the United States was represented by its ambassador to France, Jane D. Hartley.

Capitals around Europe joined the tide, as did dozens of other French cities caught up in the wave of emotion emanating from Paris. The Interior Ministry estimated that 3.7 million people had demonstrated throughout France.

Many in Paris expressed a sense that it was an unprecedented moment — one that could represent a turning point amid rising concerns about a clash of civilizations.

Photo

From a rooftop window, a view of the people who swarmed the Place de la République on Sunday for a rally described as the largest in modern French history.Credit
Peter Dejong/Associated Press

“The terrorists win if we don’t stand up and come together in this manner,” said Sharon Korman, 58, an American psychotherapist who has lived and worked in Paris for more than seven years. “Terrorism leaves us feeling afraid in our normal, daily lives. If we say, ‘I’m here anyway despite that fear,’ it makes an important statement.”

Ms. Korman added that because her mother was a Holocaust survivor, the attack on the Jewish supermarket in eastern Paris, Hyper Cacher, had made a particular impression. She said, “We are here to say ‘No, what happened this week is not O.K.' ”

Mustafa Qadir, 32, a Pakistani citizen who works as an environmental consultant in London, traveled to Paris for the march. “We cannot go on like this, living in a state of fear,” he said. “There must be liberty of expression; expression cannot be met with violence.”

At the Place de la République, where the demonstration started under a statue of Marianne, France’s symbol of freedom, many people were pressed shoulder to shoulder as helicopters circled overhead. Muslims who had heard Mr. Qadir speaking to a reporter approached to shake his hand. “They told me what happened is horrible, and does not represent Muslims,” he said.

On the Boulevard Beaumarchais, the group Reporters Without Borders formed a line, carrying photographs of the victims at Charlie Hebdo and chanting, “Liberty of expression has no religion!” People in the crowd paused silently in the street to take pictures and raise pens in homage to the dead, then burst into applause, some weeping.

The outpouring did little to stop fears of further attacks, however. Early Sunday, Hamburger Morgenpost, a German newspaper that had reprinted Charlie Hebdo cartoons lampooning the Prophet Muhammad, was the target of an apparent arson attack, the newspaper reported on its website. It said there had been no injuries. The Brussels offices of the Belgian newspaper Le Soir were also evacuated Sunday after the publication received a threat by telephone, it reported on its website.

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Agence France-Presse reported that Le Soir had also published cartoons from Charlie Hebdo.

Since the attack on Charlie Hebdo, both Mr. Netanyahu and Mr. Abbas have repeatedly condemned Islamic extremism and expressed solidarity with the French.

Jewish quarters across France have been on alert since the assault on the kosher market, which Mr. Hollande described as a “terrifying act of anti-Semitism.” Mr. Abbas and Mr. Netanyahu appeared side by side Sunday, in an unusual, if momentary, show of shared sympathy. Mr. Netanyahu drew some criticism by saying Sunday morning that “any Jew who wants to immigrate to Israel will be received here with open arms.”

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At one point, Mr. Hollande left the line of dignitaries to greet the families of the victims, as well as survivors of the attack on Charlie Hebdo. Some of the relatives and staff members were wearing white headbands that said “Charlie.” One man identified on French television as a staff member embraced Mr. Hollande for several seconds and wept.

The crowd included Jews wearing skullcaps and Muslims carrying signs reading, “I’m Charlie and Muslim” and “Not in the name of Allah.”

On the streets, people said it was impressive that world leaders had gathered, but Pablo de Gastines, a French illustrator, said the moment went “way beyond politics.” He held up a work he had painted that was inspired by Eugène Delacroix’s “Liberty Leading the People,” in which he had inserted pens and pencils as symbols of freedom.

“There is a very strong feeling here,” Mr. de Gaskins said.

Echoing many others in the crowd, Mr. de Gaskins said his main concern now was that the far-right National Front party would try to use the situation to divide the French public. “We need to make sure that they do not capture our liberty by capitalizing on what extremists did,” he said. Marine Le Pen, the leader of the National Front, was excluded from the rally.

Near the Bastille, Lilith Guillot, 23, said she had spent all day marching with friends but was dismayed that the world leaders who descended on Paris appeared to believe that the only way to fight terrorism or homegrown extremists was to spend more on security or escalate the potential for war.

She said that the people who had carried out the killings in France, and those who had committed similar acts in other European countries, had all come from deprived backgrounds. Those from France’s suburbs, she said, appeared to have gravitated to extreme Islam partly because they could never get out of the ghetto.

“What those men did was inexcusable,” Ms. Guillot said. “But all these leaders need to look at the root of the problem: integration and inclusion.”

“Until then,” she said, “nothing will change.”

Correction: January 19, 2015
An article last Monday about a huge rally in Paris to take a stand against Islamic extremism and to commemorate the 17 people killed in the terrorist attacks the previous week misstated the surname of an illustrator who attended the rally. He is Pablo de Gastines, not Gaskins.

Reporting was contributed by Maïa de la Baume, Assia Labbas and Yann Cres.

A version of this article appears in print on January 12, 2015, on Page A1 of the New York edition with the headline: Huge Show of Solidarity in Paris Against Terrorism. Order Reprints|Today's Paper|Subscribe